One of the regular questions we’re asked about Supercapacitors is about the balancing circuits when two or more cells are connected in series to increase the maximum working voltage.
All dual-cell supercapacitors require balancing to avoid an over-voltage situation occurring on any either of the cells, each of which can typically withstand around 2.5 Volts maximum across the cell.
Balancing designs are not necessarily complex and, dependent upon the application requirements can be very simple. Simple or not, it is however important to implement a balancing circuit for all dual-cell (~5v rated) or greater combinations.
Below is a summary of the common methods together with further information references which we’ve be been made aware of. Several of these can also be applied to arrays of supercapacitors connected in series.
The trade-off here is that the balancing current should be at least 10 times that of the quiescent leakage current of the supercapacitor. In some applications the increased overall current drawn from the supply isn’t so important. For example the supercapacitor might not be powered all of the time (like in an LED flash for a camera-phone) and the simplicity of the solution is the key benefit.
In other applications where the overall current drain needs to be reduced one of the following can be considered.
2. Op-Amp based Balancing circuit
This solution can be used to significantly reduce the current draw for balancing the supercapacitors.
Full details are in the following document Active_Balancing_circuit_and_information.pdf
3. Balancing with Dual Cell MOSFETs
More recently Cap-XX performed some testing using Dual Cell MOSFETs from Advanced Linear Devices.
The ALD9100xx SAB MOSFET solutions worked very well in the series of tests and the results of were recently published by ALD. This White paper is a useful reference for a very low leakage current alternative to the op amp-based active balance solution.
According to the PR on the ALD website, the dual cell MOSFETS can form a competitively priced solution in medium quantities and above.
Download the white paper here. Adv_Linear_MOSFET_Control_of_Supercapacitor_Power_Dissipation_Electronic_Products_1504.pdf
In addition to the above methods, many power managment ICs today include very low power balancing circuits for dual-cell supercapacitors. If you're close to making a choice for charge-pumps, regulators or other power-management circuits check your preferred or other manufacturers' product-line offerings. Cap-XX has worked closely with Linear Technology help with tailoring specifications to get the best results from supercapacitors.
Measuring supercapacitor paramters is almost an art form in itself!
This application note covers some of the possible measurements and methods used. Great backgorund information for all designers... Cap-XX_Applications_Note_No._1005_Supercap_Measurements.pdf
Please visit the Cap-xx products page for further information and to order.